Experimental study of turbulence decay in dense suspensions using index-matched hydrogel particles

Abstract

In the present study, a refractive-index matching (RIM) technique using hydrogel particles was developed to quantitatively measure turbulence characteristics in dense suspensions. Compared to classic RIM methods, the use of superabsorbent polymer (SAP) material significantly simplifies experimental procedures and avoids strict experimental controls, which makes the method particularly suitable for turbulence measurements in dense suspensions. Because of the high absorbency of the approximately 1 mm SAP particles, optical visibility is achieved even in dense suspensions on the order of 20% by volume. Furthermore, the small hydrogel particle diameter allows for a particle diameter-to-integral scale ratio value of 1/20. The new method is then used to reveal the flow characteristics in decaying turbulence with suspension volume fractions up to 18.4% (the measurements pass through approximately 85 hydrogel particle-water interfaces). Evidence of turbulence attenuation in suspensions is demonstrated and attributed to the inhibition of turbulence production in said suspensions. The modulations in turbulence decay are apparent even in low suspension volume fractions (2.3%), whereas the turbulence characteristics of suspensions at higher volume fractions of 9.2% and 18.4% are observed to converge on each other.